How antigens (and pathogens) enter the B cell follicles of secondary lymphoid tissues and are acquired by B cells has been a long-standing enigma. However, recent papers have identified at least two major pathways in which lymph-borne antigens enter the B cell area and are either acquired by cognate B cells or retained on follicular dendritic cells (FDC). Importantly, the nature of the antigen is a major factor. While small soluble antigens enter via a network of conduits formed by fibroblast reticular cells, larger protein immune complexes and particulate antigens are bound by subcapsular sinus macrophages and shuttled to the underlying B cells where they are captured by cognate B or bound by naive B cells via CD21 receptor. How the antigen is transferred to the FDC remains unclear. Most recently, we reported a third pathway in which lymph-borne influenza (UV-inactivated) was rapidly opsonized via mannan binding lectin and bound by macrophages lining both the subcapsular sinus and the medullary regions. Notably, macrophages are not required for humoral immunity in this pathway. Instead, dendritic cells residing in the medullary region of the LN capture virus via SIGN-R1 and appear to directly transport the viral particles to the B cell area. Whether the virus is handed off directly to FDC or to other intermediates in the follicles is not known. The overall goal of this proposal is to clarify how B cell antigen is delivered and off-loaded onto the FDC surface. Two aims are proposed. (1) Test the hypothesis that B cells transfer antigen to FDC in a 3-step process. (2) Test the hypothesis that LN resident mDC capture antigens via SIGN-R1 and traffick to FDC. )